1 model for unity but 2 files in blender?
Im Speculating that the orb wont just lay there on the plane, but maby gets some animation or interactive behavior.

The Blender file format is treated as an Prefab in Unity. Editing it further, adding scripts, components, rearranging, etc or unpacking it breaks connection and therefore the benefit of quickly updating edits from blender to unity vanishes.

Therefore I'd suggest a seperate .blend file for the floor or even exporting the active selection as .obj or .fbx and overwriting the coresponding objects in the unity asstets folder manually if you want them in the same .blend scene for better refrence (animation, modeling, etc.)